Add full compiler toolchain, libc, examples and reference docs

First substantive commit: the entire Sprinter C compiler tree on top of
the bare README+gitignore initial commit.

What's in here:
  bin/sprinter-cc        — driver script invoking SDCC + linker + mkexe
  libc/                  — Sprinter-specific libc layer over ESTEX/BIOS
                           (conio, gfx, io, mem, stdio + headers)
  runtime/               — crt0 variants (default/small/banked/minimal)
                           + heap + bank trampolines
  toolchain/             — mkexe (SprintEXE packer, C + tests)
  examples/              — 30 demo programs (gfx, file I/O, env, time, …)
  lib/Makefile           — builds the libc archive (sprinter.lib)
  docs/                  — converted Sprinter manuals + asm reference samples
  third_party/           — solid-c reference compiler dump + sdcc setup script
  release_docs/          — packaging / release notes

gitignore overhaul:
  • Drop dangerous blanket patterns: *.asm (would hide docs/samples/*.asm)
    and *.exe (case-insensitive match was hiding third_party/solid-c/*.EXE
    on macOS APFS).  Replaced with examples/*/*.{asm,exe,…} and lib/*.lib.
  • Restore tracking of toolchain/mkexe/tests/{one,big}.bin — those are
    INPUT fixtures, not build outputs.
  • Collapse the duplicated SDCC/C/Sdcc sections into one section per
    concern (build outputs / vendored / OS-junk).
  • Add .sprinter-cc-*/, build/ (catches lib/build/ too), .claude/.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

libc/io/env.c

@@ -0,0 +1,93 @@
+/*
+ * getenv / putenv via ESTEX ENVIRON ($46).
+ *
+ *   ESTEX $46 subfn 1 (getenv):
+ *     in:  HL = name (ASCIIZ), DE = output buffer (caller-owned)
+ *     out: CF=0 + A != 0  → variable found, value written into [DE..end-1]
+ *          CF=0 + A == 0  → variable not present  (note: this is the
+ *                            opposite of what DiskSyscalls.txt v1.6 docs
+ *                            claim — verified empirically and against
+ *                            solid-c's IO.ASM:763-766 implementation)
+ *          CF=1           → error, A = code
+ *          DE on exit points at one past the last byte written
+ *
+ *   ESTEX $46 subfn 2 (putenv):
+ *     in:  HL = "NAME=value" (NUL-terminated)
+ *     out: CF=1 / A = error code on failure
+ *
+ *   We hand back a pointer into a private 128-byte buffer for getenv().
+ *   Caller must copy the bytes before the next getenv() call if they
+ *   need to outlive it.
+ */
+
+#include <stdint.h>
+#include <sprinter.h>
+
+static char env_buf[128];
+
+char *getenv(const char *name) __naked
+{
+    (void)name;
+    __asm
+        push  ix
+        ;; HL = name on entry; we need to also load DE = env_buf.
+        ld    de, #_env_buf
+        ld    bc, #0x0146               ; ESTEX ENVIRON; subfn: getenv
+        rst   #0x10
+        pop   ix
+        jr    c, _getenv_err
+        or    a, a
+        jr    Z, _getenv_miss
+        ld    de, #_env_buf
+        ret
+    _getenv_err:
+        call  __errno_set
+    _getenv_miss:
+        ld    de, #0
+        ret
+    __endasm;
+}
+
+int putenv(const char *namevalue) __naked
+{
+    (void)namevalue;
+    __asm
+        push  ix
+        ld    bc, #0x0246               ; ESTEX ENVIRON; subfn: setenv
+        rst   #0x10
+        pop   ix
+        jr    c, _putenv_err
+        ld    de, #0
+        ret
+    _putenv_err:
+        call  __errno_set
+        ld    de, #-1
+        ret
+    __endasm;
+}
+
+/*  ESTEX $46 subfn 0 (sysenv):
+ *    in:  HL = caller-owned buffer
+ *    out: caller's buffer is filled with one NUL-terminated "NAME=value"
+ *         per env var, then a trailing extra NUL marks the end:
+ *           "PATH=...\0SOLID=H\0\0"
+ *    return: buf on success, -1 on error (errno set).
+ *  Buffer must be large enough for the whole environment.
+ */
+char *sysenv(char *buf) __naked
+{
+    (void)buf;
+    __asm
+        push  ix
+        push  hl                        ; stash buffer pointer (= return value)
+        ld    bc, #0x0046               ; ESTEX ENVIRON; subfn 0 = sysenv
+        rst   #0x10
+        pop   de                        ; DE = buffer pointer
+        pop   ix
+        ret   nc                        ; success: DE already has buf
+        ;; CF=1 → A = ESTEX error code.
+        call  __errno_set
+        ld    de, #-1
+        ret
+    __endasm;
+}